1. Field
The disclosure relates to an optical fiber hydrogen sensor and a method of measuring a hydrogen concentration using the same.
2. Description of the Related Art
In general, sensors for detecting hydrogen leakage (hydrogen sensors) are used to detect when hydrogen is leaked from a hydrogen-fueled vehicle or a hydrogen energy station.
However, conventional semiconductor-based hydrogen detection sensors require electric power. Thus, when electric leakage or sparks are produced, the sensors may explode on reaction with leaked hydrogen.
In order to solve the above problems, there has been a great deal of research conducted to develop an optical hydrogen sensor. Optical fiber-based hydrogen sensors using optical fibers made of a glass material have advantages in that they are electrically safe, their signal propagation velocities are high, and it is possible to detect signals at a long distance.
An optical fiber Bragg grating and a long-period optical fiber grating have been largely used for the conventional optical fiber-based hydrogen sensors.
The optical fiber Bragg grating and long-period optical fiber grating have a property that their resonance wavelengths are changed when strain or heat is applied to the gratings. When hydrogen is leaked and exposed to an optical fiber grating deposited with a hydrogen reactant (for example, palladium (Pd)), the hydrogen reactant expands by reaction with hydrogen, thereby applying strain to the optical fiber grating. As a result, the resonance wavelengths of the optical fiber grating are shifted. In this case, a hydrogen concentration may be measured by measuring a wavelength shift on the spectrum. When a tungsten oxide (WO3) is used as the hydrogen reactant, an exothermic reaction takes place on reaction of the hydrogen reactant with hydrogen. In this case, the resonance wavelengths of the optical fiber grating are shifted by means of the generated heat.
However, the conventional optical fiber grating-based hydrogen sensors have the following problems since the resonance wavelengths of the optical fiber grating are shifted by two physical variables such as temperature and strain. First, when palladium is used as the hydrogen reactant, a change in outside temperature during detection of hydrogen affects the resonance wavelengths of the optical fiber grating, which makes it difficult to determine a change in resonance wavelength due to volume expansion of the palladium. Meanwhile, when a tungsten oxide is used as the hydrogen reactant, and external strain is applied to the optical fiber grating in a longitudinal direction, it is difficult to identify only a change in resonance wavelength due to an exothermic reaction of the tungsten oxide. Also, since the optical fiber grating is manufactured with long-term irradiation with ultraviolet rays, it may be easily broken due to external stress applied on a transverse axis and it is cumbersome to manufacture.